EDIT ****THIS ACHIEVEMENT IS NOW OBTAINABLE**** 6/30/2016
The map i have done this on the most, before it was fixed, is Temple of Anubis. The first area is a really hard choke point and if your team can hold it long enough its pretty easy.
You can get this achievement in competitive or in Quick play. Thanks to Fooga!
**Original post
Looks like this is currently glitched. Have done this about 10 times now on different maps. Will delete post when it is fixed**